Orthogonal frequency division multiplexing (OFDM) is a scheme for communicating digital data over a wireless channel. OFDM effectively mitigates the intersymbol interference (ISI) caused by channel time spread and only utilises simple frequency domain channel equalization. Due to these advantages, OFDM has been widely used in wireless personal, local, and metropolitan area networks (WPANs, WLANs, and WMANs) and digital audio and video broadcasting services (DAB/DVB). OFDM is also the strongest candidate scheme for future generation wireless mobile communication systems.
Multiple-input multiple-output (MIMO) techniques use multiple antennas at the transmitter and/or receiver ends of a wireless communication link to increase the system spectral efficiency and/or enhance the received radio signal quality. MIMO systems can operate in different transmission modes, such as spatial diversity, spatial multiplexing, and beamforming, depending on the signal correlations among antenna elements.
Spatial diversity refers to the use of multiple antennas to improve the link quality between the transmitter and the receiver. If the antenna elements are spaced sufficiently apart (or configured by some other means such as changing polarization and radiation patterns) so that uncorrelated signal paths can be formed, the link quality parameters such as the signal-to-noise ratio (SNR) can be improved by advanced signal processing algorithms implemented at the transmitter and/or the receiver end. Spatial multiplexing exploits the scattering properties of the wireless MIMO channels by transmitting multiple data streams, called substreams, in parallel on multiple antennas to increase link capacity. Like spatial diversity, spatial multiplexing requires uncorrelated MIMO channels, as well as high SNR. Beamforming allows spatial access to the radio channel by means of focusing the energy into some desired directions and nulling the others, leading to an increase of the average SNR. In beamforming mode, the MIMO channel structure and scattering properties are not exploited to define uncorrelated channels, but to obtain an equivalent single channel with improved properties.
Each of the above MIMO modes relies on a certain amount of available channel state information (CSI) at the transmitter and/or the receiver end. The CSI can be made available at the transmitter through feedback from the receiver or obtained based on estimation of the receive channel.
It is advantageous for a MIMO system to be able to adapt its parameters, signal processing algorithms or even physical antenna configuration according to different channel conditions and system requirements. Existing adaptive MIMO systems utilise linear precoding, antenna selection, or switching between MIMO transmission modes.
Linear precoding, commonly combined with space-time coding (transmitting a data symbol across different transmit antennas and time slots to enable the data symbol to experience different fading effects so that the received signal quality can be improved after diversity combining), is a technique by which the decoding complexity can be dramatically reduced. That is, joint maximum-likelihood (ML) decoding of the transmitted symbols can be decoupled into symbol-by-symbol decoding via linear precoding. An adaptive MIMO transmitter with linear precoding uses a linear filter (implemented via matrix multiplication) designed by making use of information about the channel conditions and/or propagation properties. The design of the linear filter can be based on a selected performance criterion. The main advantage of the linear precoding approach is that it does not have to track fast fading but only the slowly varying antenna correlations, which can be obtained from a low-rate feedback link or can be derived based on channel estimation using the reciprocity principle.
Antenna selection uses only a subset of the available transmit and/or receive antenna elements to reduce the system complexity and cost, while meeting some specified performance criteria. There are two kinds of antenna selection techniques. One is deterministic antenna selection, by which different sets of antenna elements are selected according to the instantaneous channel state, and the optimal set is determined every time the channel changes. The other is statistical antenna selection, which is based on second-order channel statistics, when spatial multiplexing or space-time coding techniques are used over the wireless link.
MIMO transmission mode switching between spatial multiplexing and spatial diversity achieves a trade-off between data rate and reliability under different antenna correlations. The switching can be based on the instantaneous channel state, which requires a low-rate feedback channel from the receiver to the transmitter. In order to maximize system throughput, it is also possible to switch between spatial multiplexing in low element correlation conditions and beamforming in high element correlation conditions. Mode switching requires minimal feedback information since it relies on only two channel statistics, the average SNR and the spatial correlation between antennas.
In a MIMO system, signals are transmitted not only through different links among transmit and receive antenna elements but also through multiple paths with different time delays. The multipath transmission will lead to frequency-selective fading in OFDM systems, which worsens system performance.